[25.02] EGRET Detection of Gamma Rays from the Moon

The Energetic Gamma Ray Experiment Telescope (EGRET) on the
Compton Gamma Ray Observatory has detected gamma rays from
the Moon as it passed through the instrument field of view
several times between 1991 and 1994. The average flux,
(4.7 \pm 0.7) \times 10^-7 ph(>100 MeV)/cm^2s,
and the energy spectrum of the lunar gamma radiation are
consistent with a model of gamma ray production by cosmic ray
interactions with the lunar surface, and the flux varies as
expected with the solar cycle. Although the same processes
may occur on the Sun, EGRET does not detect the quiet Sun.
The upper limit, 3.0 \times 10^-7 ph(>100 MeV)/cm^2s,
does not contradict calculations of the expected solar
gamma-ray flux. Thus, in high-energy gamma rays, the Moon
is brighter than the quiet Sun.

The author(s) of this abstract have provided an email address for comments about the abstract: djt@egret.gsfc.nasa.gov